Cabin air pressure & alt static source question

cowman

cowman

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Cowman
Ok I'm studying for the instrument written and I came across this question and the answer/explanation in the book is bugging me.

Here's the question
If, while in level flight, it becomes necessary to use an alternate source of static pressure vented inside the airplane, which of the following should the pilot expect?

A. The vertical speed to show a climb

B. The vertical speed to momentarily show a descent

C. The altimeter to read higher than normal.
Click to expand...

I thought the altimeter would read LOWER than normal but according to the book the correct answer is C, with the explanation that "the pressure within an unpressurized cockpit is slightly lower than the pressure outside the airplane because of the Venturi efffect...."

Now, that explanation makes perfect sense to me except that I thought the air pressure in my cabin was HIGHER than the outside air pressure. I think this because in my plane (Archer II) I have a small "storm" window on the pilot side that opens inward. In flight this window is very difficult to open because(I thought) positive pressure in the cabin was pushing and holding it closed(outward). I had always assumed this was because of air being rammed in through the fresh air vents on the wing leading edges and tail creating a slightly higher pressure.

What I can't figure out is why, if the book is right and cabin air pressure is in fact lower why my storm window wouldn't in fact fly open when I undo the latch rather than needing to be pried open with fingernails.

Can someone explain where my logic or perhaps the book explanation has failed me?
 
The cabin pressure is lower than ambient because the pressure around it is lower yet, drawing air out of the cabin.

Just to put some fake numbers to it...your altimeter reads 5000 feet using the normal static system. It might read 5100 feet using the alternate system. If you ported it just outside your storm window, it might read 5200 feet.
 
The air has to go around the outside of the fuselage - so it accelerates and the pressure on the surface is lower as it gets to the rear of the airplane it gets pretty much back to static.
You cabin leaks here and there and ends up lower than static pressure - how much lower depends on where the biggest leaks are. Right at the front of the cabin where the cross sectional area is the highest will be the lowest external pressure - that is where your storm window be. So if the cabin is at something less than static pressure, but not as low as the lowest pressure around (think big leak where the holes to let the elevator linkage out for cabin pressure) then, your window is hard to open.
 
The airflow around the fuselage, cabin vent window, has accelerated creating a lower surface pressure, just like the top of your wing. That lower surface pressure is pulling your vent window closed. You have to pull against that to open the window inflight, it is not internal pressure that is holding it closed.

Leave your fuel cap off in flight and it vents fuel out of the opening. It is not internal pressure forcing the fuel out, but the “suction” of the low surface pressure. Sam Ting on the vent window.
 
They aren't exactly the same. It is differential pressure, higher pressure inside the aircraft and lower surface pressure (from acceleration) on the outside surface working together. You will notice that the moment the window opens slightly the differential pressure drops significantly making window movement much easier.
 
Last edited:
GlennAB1 said:
You will notice that the moment the window opens slightly the differential pressure drops significantly making window movement much easier.
Click to expand...
You are also likely to see a couple miles per hour in indicated airspeed change if you are using the cabin air as your static source as you open / close the window.
 
You're lucky to have such a well sealed airplane!
 
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